1. Field of the Invention
The invention relates to a DC-DC converter; in particular, to a power converter capable of converting the decreased input voltages into expected output voltages and an operating method thereof.
2. Description of the Prior Art
In general, an input voltage inputted to a power converter should be certain extent higher than an expected output voltage to smoothly generate the expected output voltage. However, because the input voltage is often decreased with time, the output voltage generated by the power converter fails to reach the expected output voltage.
As shown in FIG. 1, under the constant-on-time (COT) pulse width modulation (PWM) structure, an amplifier 10 of a power converter 1 receives a feedback voltage VFB (related to an output voltage) and a reference voltage Vref. Once the feedback voltage VFB is lower than the reference voltage Vref, it means that the output voltage VOUT generated by the power converter 1 is obviously too low. Therefore, a PWM unit 12 outputs a PWM signal PWM having a fixed on-time (Ton) to an output stage 14 to charge an output capacitor C to increase the output voltage VOUT.
As shown in FIG. 2, each on-time Ton in the PWM signal PWM is followed by an off-time Toff. The combination of an on-time Ton and an off-time Toff is defined as a period of the PWM signal PWM. At first (before the time ta), because the input voltage VIN is high enough and has a fixed value, the on-time Ton/the off-time Toff is fixed and repeatedly appear. With the decreasing of the input voltage VIN from the time ta, although the period of the PWM signal PWM won't be changed, the on-time Ton of the period is increased and the off-time Toff of the period is decreased to keep the output voltage VOUT at the expected value instead of becoming lower. However, after the input voltage VIN is decreased to a threshold voltage Vth at a time tb, the period of the PWM signal PWM is still fixed and the off-time Toff of the period can't be further decreased. Therefore, the on-time Ton be fixed and fail to provide enough charging time, and the output voltage VOUT be decreased with the decreasing of the input voltage VIN. It seriously affects the voltage conversion efficiency of the power converter 1.
Therefore, the invention provides a power converter and an operating method therefore to solve the above-mentioned problems occurred in the prior arts.